1. Field of the Invention
The present invention relates to an expansion dowel, in particular, to an undercut self-cutting dowel including an expansion sleeve having a rear section provided with an inner thread, an opposite expansion section with a plurality of expansion tabs, and a through-bore, with the dowel further including an expansion member provided at a front end of the expansion sleeve and retained in the sleeve bore, with the expansion member projecting beyond the front end of the expansion sleeve, and the expansion tabs expanding radially outwardly upon the expansion sleeve being driven over the expansion member.
2. Description of the Prior Art
For fastening of constructional parts to a constructional component made, e.g., of concrete or stone, as a rule, expansion dowels are used. A conventional expansion dowel includes essentially an expansion sleeve having an expansion region and an expansion member. Upon the relative movement of the expansion sleeve and the expansion member, the expansion region of the expansion sleeve expands radially. As a result of the radial expansion of the expansion region of the expansion sleeve, the expansion dowel becomes force lockingly or form lockingly anchored in a bore formed in the constructional component. In cases, where small axes and edge distances must be maintained, primarily, formlockingly anchorable expansion dowels are used. The attachment of this type forms a so-called undercut system. To form an undercut fastening or attachment system, first, an undercut is formed in the vicinity of the bottom of the bore in which the expansion dowel is received. Then, an anchor rod with an expansion sleeve mounted thereon is inserted into the bore. The expansion tabs of the expansion sleeve expand radially upon relative movement between the expansion sleeve and a cone head formed at the front end of the anchor rod and widening in the setting direction. The expansion tabs, which engage in the undercut, form a formlocking connection of the dowel with the constructional component.
In accordance with one embodiment of an undercut attachment system, the expansion dowel is being anchored without first forming an undercut with a special tool. With this embodiment, a formlocking connection is formed automatically during the setting process of the dowel. The formation of the formlocking connection takes place as a result of rotation of the expansion sleeve during the setting process, with the sleeve being simultaneously driven onto the widening in the setting direction, cone head by axial blows imparted by a hammer mechanism. The expansion tabs of the rotatable expansion sleeve are provided at their free ends with cutters which mill an undercut in the bore wall during the setting process. At the end of the setting process, the radially expanded expansion tabs become formlockingly anchored in the undercut in the bore wall during the setting process. In addition to the undercut self-cutting expansion dowel which forms an undercut as a result of the rotation of the expansion sleeve, also known are undercut attachment systems with which formlocking connection is achieved without the rotation of the expansion sleeve. For the formation of an undercut, it is sufficient to displace the expansion sleeve over a conical expansion member by axial blows. During the displacement of the expansion sleeve, the cutters, which are provided at the free ends of the expansion tabs, form an undercut by chiseling the bore wall, with the chiseled wall material being simultaneously removed, and with the expansion tabs being formlockingly anchored in the undercut at the end of the setting process.
The undercut attachment systems includes primarily an anchor rod provided with a head portion widening in the setting direction and an expansion sleeve displaceable along the anchor rod. However, it is also desirable to have formlockingly fastening systems with no connection parts projecting above the upper surface of a constructional component in which an expansion dowel is anchored. With these systems, it becomes possible, e.g., to provide an attachment point at which a constructional part can be attached at a later time. Such undercut attachment systems includes an expansion sleeve with an inner thread and having a through-bore and a plurality of expansion tabs which are provided in the expansion region of the sleeve and are separated by axial slots. At a front end of the expansion sleeve, there is provided an expansion member which has a smaller diameter section projecting into the sleeve bore, whereby the expansion member is held by the expansion sleeve. For formlocking anchoring of a such undercut self-cutting expansion dowel, the expansion sleeve is driven by axial blows over the expansion member which is supported against the bottom of a bore formed in a constructional component. The cutters, which are provided at free ends of the expansion tabs of the expansion sleeve, chisel the bore wall, forming an undercut. For axially driving the expansion sleeve over the expansion member, usually, axial blows are applied directly to the rear end of the expansion sleeve of the dowel, which is inserted in the constructional component bore, with a hammer. As a result of application of axial blows directly to the rear end of the sleeve, the inner thread at the entrance can be damaged. Further, with the expansion dowels having external conical expansion members, there is no possibility to control during the setting process or thereafter whether the expansion tabs of the expansion sleeve expanded at the required circumference. It should be noted that both insufficient expansion and over-expansion of the expansion tabs negatively influence the desired retaining or holding value of the expansion dowel. Therefore, for anchoring of impact-driven expansion dowels provided with an expansion member retained at the front end of the expansion sleeve, special tools are used for preliminary driving the expansion member to the required circumference. However, such tools cannot be used for the expansion dowels with an expansion sleeve having an inner thread and an outwardly extending expansion member.
Accordingly, an object of the present invention is to provide an expansion dowel, in particular, an undercut self-cutting expansion dowel, having an expansion sleeve and an externally located expansion member and which can be anchored without any danger of the inner thread of the expansion sleeve, in particular, of the thread entrance, being damaged during the anchoring process.
Another object of the present invention is to provide an expansion dowel, in particular, an undercut self-cutting dowel of the above-described type, which can be anchored without use of specially formed setting tools.
A further object of the present invention is to provide an expansion dowel, in particular an undercut self-cutting dowel, of the above-described type, with which an operator can ascertain both during the setting process and thereafter whether the expansion tabs of the expansion sleeve expanded radially at a necessary circumference and whether the dowel is correctly set.